An Analog Comparator as a Pseudo-Light Pen for Computer Displays

An Analog Comparator Interrupt Device signals the computer central processor whenever the computer display unit deflection voltages cause the CRT beam to fall within a small square surrounding a previously specified display coordinate. This provides the display with the sensing capability of a light pen that is necessary if a RAND tablet or other position input device is to be used to point at display picture elements in graphics applications. The X and Y deflection voltages corresponding to the cursor or pointer are stored and then compared with subsequent X and Y input voltages. An interrupt output is produced when a match occurs within certain limits. Operational amplifiers and field-effect transistor switches are connected in a simple feedback loop to provide storage and comparison. A diode net and threshold detector produce the output signal. This unit is now being used with the Lincoln Laboratory TX-2 computer curve-drawing display unit.